Make find_user_id easier to use for non-technical users

[JiPaix]

why

The original find_user_id.py script worked, but it was rough around the edges
for anyone who isn't a developer.

fixes

• the scan kept running even after finding a valid ID, wasting time and
  requests against the server.

changes

• Clearer configuration
  • renaming USERNAME to EMAIL and CNT_ID to CUSTOMER_REF
  • added a large header comment at the top explaining exactly what each value is
    and where to find it (invoice, customer portal, etc.).
• Configurable search range: replaced the hardcoded range(490000, 500000)
  with SEARCH_START / SEARCH_END constants, with comments showing how to
  split a large scan into several smaller runs.
• Live progress reporter: a background thread now prints
  checked / rate / elapsed / ETA every 5 seconds, so users can see the
  scan is actually working and estimate how long it will take.
• Startup logging: prints the resolved config (email, customer ref,
  thread count, re-login interval) before starting, so misconfigurations are
  obvious right away.
• Early stop on hit: as soon as a valid user ID is found, the script prints
  the result + final stats and terminates the whole process via os._exit(0),
  instead of continuing to scan the rest of the range.

not changed

• Authentication flow, request structure, threading model, and the underlying
  endpoint are untouched.
• No new dependencies.

optional

added a French-localized version of the script
(find_user_id_fr.py) for users who are more comfortable reading the setup
instructions in French. Behavior is identical.

[jugla]

Thanks a lot for the improvment